Unit 2 data link control

1. Unit 2
Data Link Control

2. Topics to be covered in this unit . . . . . . . . .
 Framing
 Flow Control
 Error Control
 Protocols
~ for Noiseless ( ideal ) Channels
~ for Noisy (practical or real ) Channels
 HDLC Protocol

3. 2-1 FRAMING
 The data link layer needs to pack bits into frames,
so that each frame is distinguishable from another.
 Our postal system practices a type of framing.
The simple act of inserting a letter into an envelope
separates one piece of information from another;
the envelope serves as the delimiter.
Topics discussed in this section:
a) Fixed-Size Framing
[ The size itself acts as delimiter or frame boundary ]
b) Variable-Size Framing
[ special character or bit pattern called flag is used as delimiter ]

4. A frame in a character-oriented protocol
A frame in a bit-oriented protocol

5. Byte stuffing and unstuffing
Note
Byte stuffing is the process of adding 1 extra byte
whenever there is a flag or escape character in the text.

6. Bit stuffing and unstuffing
Bit stuffing is the process of adding one extra 0 whenever
five consecutive 1s follow a 0 in the data, so that the receiver
does not mistake the pattern 01111110 for a flag.
Note

7. 2-2 FLOW AND ERROR CONTROL
The most important responsibilities of the data link
layer are flow control and error control. Collectively,
these functions are known as data link control.
Topics discussed in this section:
a) Flow Control
b) Error Control

8. Flow control refers to a set of procedures
used to restrict the amount of data
that the sender can send before
waiting for acknowledgment.
Error control in the data link layer is
based on automatic repeat request (ARQ),
which is the retransmission of data.

9. 2-3 PROTOCOLS
 The protocols are normally implemented in software by using one
of the common programming languages.
To make our discussions language-free, we have written in
pseudo code a version of each protocol that concentrates mostly on
the procedure instead of delving into the details of language rules.

10. 2-3 a) NOISELESS CHANNELS
 Let us first assume we have an ideal channel in
which no frames are lost, duplicated, or corrupted.
 In other words, the channel is error free and as a
result we don’t require any error control
mechanism in the corresponding protocols.
 We introduce two protocols for this type of channel.
Topics discussed in this section:
a) Simplest Protocol
b) Stop-and-Wait Protocol

11. 1. Simplest Protocol
We assume that the receiver can immediately handle any frame
it receives with a processing time that is small enough to be
negligible.
 In other words, the receiver can never be overwhelmed with
incoming frames.
 Hence there is no need for flow control in this scheme.

12. a) Design

13. b) Algorithms
Reciever Site Sender Site

14. c) Flow Diagram [ for Simplest Protocol ]

15. 2. Stop-and-Wait Protocol
 If the data frames arrives at the receiver site faster than they can
processed, the frames must be stored until their use.
 Normally, the receiver does not have enough storage space, especially
if it is receiving data from many sources.
 Hence to prevent the receiver from becoming overwhelmed with
incoming frames, we somehow need to tell the sender to slow down. As
such there must be feedback from the receiver to the sender. In other
words we need to employ a flow control mechanism in the protocol.
 Acknowledgement (ACK) frames that are auxiliary frames help in this
regard.
 The protocol is called Stop-and-Wait protocol because the sender
sends one frame, stops until it receives confirmation from the receiver
(okay to go ahead ) and then sends the next frame.

16. a) Design
Note :The regions highlighted with red shade indicate additions to the previous protocol.

17. b) Algorithms
Sender Site

18. Receiver Site

19. c) Flow Diagram [ for Stop & Wait Protocol ]

20. 2-3 b) NOISY CHANNELS
Although the Stop-and-Wait Protocol gives us an idea
of how to add flow control to its predecessor, noiseless
channels are nonexistent. We discuss three protocols
in this section that use error control.
Topics discussed in this section:
a) Stop-and-Wait Automatic Repeat Request
b) Go-Back-N Automatic Repeat Request
c) Selective Repeat Automatic Repeat Request

21. 3. Stop-and-Wait ARQ Protocol
 Error correction in Stop-and-Wait ARQ is done by
keeping a copy of the sent frame and retransmitting of
the frame when the timer expires.
 In Stop-and-Wait ARQ, we use sequence numbers to
number the frames. The sequence numbers are based
on modulo-2 arithmetic.
 In Stop-and-Wait ARQ, the acknowledgment number
always announces in modulo-2 arithmetic the sequence
number of the next frame expected.

22. a) Design

23. (continued) . . . . .
b) Algorithm
Sender Site

24. (continued) . . . . . .

25. Receiver Site

26. c) Flow Diagram
Data Frame lost in the
middle of transmission.
Acknowledgement Frame lost
in the middle of transmission.

27. Efficiency & Pipelining
 The Stop & Wait ARQ discussed in the previous section is
very inefficient if our channel is thick and long. By thick it is
meant that the channel has a large bandwidth; by long, we
mean the round-trip delay is long.
 The product of these two is called the bandwidth-delay
product. The bandwidth-delay product is a measure of the
number of bits we can send out of our system while waiting
for news from the receiver.
 Pipelining is a technique in which a task is often begun before
the previous task has ended. This is known as pipelining.
 If we employ pipelining concept then several frames can be
sent before we receive news about the previous frames.
 Pipelining improves efficiency of the transmission if the
number of bits in transition is large with respect to the
bandwidth-delay product.

28. Efficiency : 1st Numerical
1) Assume that, in a Stop-and-Wait ARQ system, the bandwidth of
the line is 1 Mbps, and 1 bit takes 20 ms to make a round trip. What
is the bandwidth-delay product? If the system data frames are 1000
bits in length, what is the utilization percentage of the link?
Solution
The bandwidth-delay product is
The system can send 20,000 bits during the time it takes for the
data to go from the sender to the receiver and then back again.
However, the system sends only 1000 bits. We can say that the link
utilization is only 1000/20,000, or 5 percent. For this reason, for a
link with a high bandwidth or long delay, the use of Stop-and-Wait
ARQ wastes the capacity of the link.

29. Efficiency : 2nd Numerical
What is the utilization percentage of the link in
previous numerical example, if we have a protocol that
can send up to 15 frames before stopping and
worrying about the acknowledgments?
Solution
The bandwidth-delay product is still 20,000 bits. The
system can send up to 15 frames or 15,000 bits during a
round trip. This means the utilization is 15,000/20,000, or
75 percent. Of course, if there are damaged frames, the
utilization percentage is much less because frames have
to be resent.

30. 4. Go-Back-N ARQ Protocol
 To improve efficiency of transmission, we should have a
protocol in which multiple frames must be in transition
while waiting for acknowledgement.
 In other words, we need to let more than one frame be
outstanding to keep the channel busy while the sender is
waiting for acknowledgement.
 Go-Back-N is one such protocol. In this protocol
we can send several frames before receiving
acknowledgements; we keep a copy of these frames
until the acknowledgements arrive.

31. i) Sequence Numbers
In the Go-Back-N Protocol, the sequence
numbers are modulo 2m, where m is the
size of the sequence number field in bits.
For example,
~ if m=2 then the sequence numbers will range from 0 to 3.
( 0, 1, 2, 3 )
~ If m=4 then the sequence numbers will range from 0 to 15.
( 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 )

32. ii) Windows :
a) Send Window :

33. The send window is an abstract concept
defining an imaginary box of size 2m − 1
with three variables: Sf, Sn, and Ssize.
The send window can slide one
or more slots when a valid
acknowledgment arrives.

34. b) Receive Window :

35. The receive window is an abstract concept defining an
imaginary box of size 1 with one single variable Rn.
The window slides when a correct frame has arrived;
sliding occurs one slot at a time.

36. a) Design
Multiple Frames

37. Window size for Go-Back-N ARQ
Note
Will work correctly
Will work erroneously
In Go-Back-N ARQ, the size of the send window must be less than 2m;
the size of the receiver window is always 1.

38. (continued . . . . . . . . )
b) Algorithm
Sender Site

39. (continued . . . . . . . . . )

40. Receiver Site
No significant changes here compared to previous protocol

41. c) Flow Diagram [ for Example 1 ]
Acknowledgement Frame-2 lost.
Note : ACK3 acts as cumulative acknowledgement for both Frames 1 & 2.

42. Flow Diagram [ for Example 2 ]
Data Frame lost in the
middle of transmission.
Retransmission of
Frames 1, 2 and 3.

43. Stop-and-Wait ARQ is a special case of
Go-Back-N ARQ in which the size of the
send window is 1.
Note

44. 5. Selective Repeat ARQ Protocol
 Go-Back-N ARQ simplifies the process at the receiver
site. The receiver keeps track of only one variable, and
there is no need to buffer out-of-order frames; they are
simply discarded.
 However, this protocol is very inefficient for a noisy link.
In a noisy link a frame has higher probability of damage,
which means the resending of multiple frames. This
resending uses up the bandwidth and slows down the
transmission.
 For noisy links, there is another mechanism that does
not resend N frames when just one frame is damaged;
only the damaged frame is resent. This mechanism is
called Selective Repeat ARQ. It is more efficient for noisy
links, but processing at the receiver is more complex.

45. Windows
a) Send window for Selective Repeat ARQ
b) Receive window for Selective Repeat ARQ

46. a) Design

47. Selective Repeat ARQ, window size
Will work erroneously
Will work correctly
Note : In Selective Repeat ARQ, the size of the sender and
receiver window must be at most one-half of 2m.

48. b) Algorithms
Sender Site
(continued . . . . . . . . . )

49. (continued . . . . . . . . . )

50. Receiver Site
(continued . . . . . . . . . )

51. (continued . . . . . . . . . )

52. Delivery of data in Selective Repeat ARQ

53. c) Flow diagram
Data Frame lost in the
middle of transmission.
Frame 2 is received and
NAK is sent for Frame 1
Note : ACK4 acts as cumulative acknowledgement for Frames 1, 2 and 3.

54. Design of piggybacking in Go-Back-N ARQ

55. 11-6 HDLC
High-level Data Link Control (HDLC) is a bit-oriented
protocol for communication over point-to-point and
multipoint links. It implements the ARQ mechanisms
we discussed in this chapter.
Topics discussed in this section:
a) Configurations and Transfer Modes
~ Normal Response Mode [NRM]
~ Asynchronous Balanced Mode [ABM]
b) Frames
c) Control Field

56. i) Normal Response Mode [ NRM ]

57. ii) Asynchronous Balanced Mode [ ABM ]

58. b) HDLC Frames
11.58

59. c) Control Field Format ( for the different frame types )

60. U-frame control command and response

61. Numerical 1 on connection & disconnection in HDLC
Figure in the next slide shows how U-frames can be used
for connection establishment and connection release.
Node A asks for a connection with a set asynchronous
balanced mode (SABM) frame; node B gives a positive
response with an unnumbered acknowledgment (UA)
frame. After these two exchanges, data can be transferred
between the two nodes (not shown in the figure). After
data transfer, node A sends a DISC (disconnect) frame to
release the connection; it is confirmed by node B
responding with a UA (unnumbered acknowledgment).

62. Example of connection and disconnection

63. Numerical 2 on piggybacking without error in HDLC
Figure in the next slide shows an exchange using piggybacking. Node
A begins the exchange of information with an I-frame numbered 0
followed by another I-frame numbered 1. Node B piggybacks its
acknowledgment of both frames onto an I-frame of its own. Node B’s
first I-frame is also numbered 0 [N(S) field] and contains a 2 in its
N(R) field, acknowledging the receipt of A’s frames 1 and 0 and
indicating that it expects frame 2 to arrive next. Node B transmits its
second and third I-frames (numbered 1 and 2) before accepting
further frames from node A. Its N(R) information, therefore, has not
changed: B frames 1 and 2 indicate that node B is still expecting A’s
frame 2 to arrive next. Node A has sent all its data. Therefore, it
cannot piggyback an acknowledgment onto an I-frame and sends an
S-frame instead. The RR code indicates that A is still ready to receive.
The number 3 in the N(R) field tells B that frames 0, 1, and 2 have all
been accepted and that A is now expecting frame number 3.

64. Example of piggybacking without error

65. Numerical 3 on piggybacking with error in HDLC
Figure in the next slide shows an exchange in which a
frame is lost. Node B sends three data frames (0, 1, and
2), but frame 1 is lost. When node A receives frame 2, it
discards it and sends a REJ frame for frame 1. Note that
the protocol being used is Go-Back-N with the special use
of an REJ frame as a NAK frame. The NAK frame does
two things here: It confirms the receipt of frame 0 and
declares that frame 1 and any following frames must be
resent. Node B, after receiving the REJ frame, resends
frames 1 and 2. Node A acknowledges the receipt by
sending an RR frame (ACK) with acknowledgment
number 3.

66. Example of piggybacking with error